Hydrojets, which are preferably arranged on the bow side in the bottom area, have proved to be successful for improving maneuverability especially in watercraft used in shallow waters (e.g., in inland waters with varying water levels). Such hydrojets, also called bow jet units, comprise a housing, which can be installed in the bottom of the particular watercraft and which contains at least one propeller (or a pump impeller), which introduces energy into the water fed in via a bottom-side housing intake and releases the water, for example, via an elbow and channels in their direction, or steers it all round through at least one discharge opening, which is flush with the bottom and can usually be pivoted by 360° under the bottom of the boat.
The axis of rotation of the propeller or of the pump impeller extends either in the horizontal direction or in the vertical direction in prior-art hydrojets.
In case of vertical arrangement of the propeller, i.e., rotation around a horizontal transverse axis in a cross jet tunnel, it shall be borne in mind that the water line should be about half the propeller diameter above the vertex of the tunnel to avoid the inrush of air and the drop in thrust caused thereby. This leads to the drawback that the watercraft in question has a relatively great draft and requires a corresponding depth of the shipping channel in order to be able to manoeuver without risk and efficiently.
Besides this, hydrojets with vertical arrangement of the propeller are also known, which have a bottom intake according to the principle of usual axial jets in order to reduce the draft. Because the suction behavior of axial-flow pumps allows a limited emersion of the propeller blades only, a draft ratio that depends on the function is to be ensured.
A hydrojet for the bow control of a watercraft with horizontally aligned propeller axis of rotation is known from EP 0 024 443 A. In this prior-art hydrojet, the discharge opening for the water jet is located in the direction of travel in front of the intake opening, such that only a brake thrust and a maneuvering thrust for the support of the helm on the stern side can be produced. In contrast, an efficient driving thrust cannot be produced with a hydrojet designed in this way and arranged in the ship's bow, because a propulsion of the thrust jet, generating and in this respect with a relatively flat slope in the forward direction crosses or strongly swirls the suction flow directed sharply upwards there. The turbulences and suction effects produced thereby increase the intake losses sharply and thus drastically reduce the thrust development in this control zone. The propeller water drawn in in this case likewise contributes to thrust reduction, because it is accelerated less with higher intake velocity and thus produces less thrust.
Even though hydrojets with vertically directed axis of rotation require a relatively small draft for air-free operation for the propeller (or pump impeller) arranged horizontally above a bottom intake, they do have, among other things, the drawback that in case of shallow depth of water (i.e., at less than, e.g., 50 cm of water under the keel), such drives generate a strong wake due to the suction effect directed directly toward the bottom, which wake increases the resistance of the boat and compromises the development of thrust or lets it collapse altogether when increased amounts of foreign bodies sucked in clog the protective grid, which is usually present. Furthermore, the risk for damage increases, because small foreign bodies can pass through the protective grid like a sieve and can increasingly enter between the blades.
In addition, it was found that in case of hydrojets with vertically directed axis of rotation of the propeller, the flow to be deflected into the vertical suction area begins to be broken down beginning from a certain velocity of travel with increasing speed, which will then lead to a drastic drop in thrust.
For driving via a horizontally arranged motor, hydrojets with vertical axis of rotation of the propeller require an angular gear of their own. For driving by means of internal combustion engines, a marine reversing gear, which as a second gear increases the mechanical losses and the cost of the unit, is also necessary for coupling and changing over the direction of rotation (for example, for flushing the protective grid).
Finally, a hydrojet for the bow control of boats has become known from CH 551 311 A, in which a propeller is provided, whose axis has a slope angle ≦45° in relation to a horizontal base. As in the case of the above-mentioned EP 0 024 443 A, the discharge opening for the hydrojet in the direction of travel is located in front of the intake opening. The self-closing blades arranged there behind the propeller as well as the rudder action which is only efficient in the jet direction rule out a use of this jet for thrust production.